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Corresponding Author: Barbara Bielowicz; e-mail: bbiel@agh.edu.pl
1 AGH University of Science and Technology in Kraków, Poland; ORCID iD: 0000-0002-8742-5890; e-mail: bbiel@agh.edu.pl
DOI: 10.24425/gsm.2020.132561
BARBARA BIelOwICz1
changes in the spontaneous combustion tendency
of humic coals according to their petrographic composition
and physico-chemical properties
introduction
Currently, due to the worldwide trade of coal and the resulting need for long-term storage of this raw material, the problem of self-heating and self-ignition is particularly important. In addition, the question of self-ignition and activation energy is of importance when taking clean coal technologies such as the gasification process into account. The spontaneous com-bustion is the tendency of coal, in particular crushed coal, to self-ignition intensified by heat from the decomposition (in moisture) of inclusions of pyrite and chalcopyrite. The studies on the spontaneous combustion of coal have a long history. This research problem has been ad-dressed, among others, by: (Berkowitz and Schein 1951; Bhattacharyya 1972; Falcon 1986; Singh 2013; Thomas and Fariborz 2000; Kataka et al. 2018). The self-heating of coal leading to spontaneous combustion poses a serious threat during its mining, transport and storage
(Misra and Singh 1994). According to previous studies, low-rank coals are more prone to
type (Clemens et al. 1991; Panigrahi and Sahu 2004), particle size and climate (Misra and Singh 1994). In the case of lower rank coals, a high percentage of reactive macerals (vitrinite and liptinite) may increase the tendency for heating (lyman 2001). Moreover, the tendency for heating is correlated with higher moisture, net calorific value and higher oxygen content. In addition, the potential of higher-sulfur coals for increased heating stems from the fact that pyrite oxidation is exothermic. The pyrite oxidation was believed to be a major factor in spontaneous combustion; however, it is currently considered to be less important than other oxidizing characteristics. Pyrite contents above 2% are generally required to have any influence. Coal richer in reactive macerals, particularly liptinite and vitrinite, is more prone to self-heating than inertinite-rich macerals (Beamish et al. 2001). Therefore, the main fac-tors affecting the spontaneous combustion of coal include: the rank of coal, oxygen content, the flow rate of the air, particle size, and the moisture content. Special attention should be paid to works focusing on the adsorption of gases released during the self-heating of coal (Cygankiewicz et al. 2006; Zarębska et al. 2012; Dudzińska and Cygankiewicz 2015; Dudzińska 2019). The above authors suggested that the fire gases from the self-heating center can be adsorbed by bituminous coals; this can result in a lower concentration of gas and a change of fire indices. The need to improve the spontaneous combustion indicators contained in the existing standards was also highlighted by Słowik (Słowik 2019), who drew attention to the influence of the temperature of the coal seam.
1. methods
A total of 29 coal samples from Polish deposits, including 11 low-rank coal from the Polish lowlands, 16 bituminous coal from the Upper Silesian Coal Basin and lublin Coal Basin, and one anthracite sample from the lower Silesian Coal Basin were used in the study. The names of mines or opencasts, age, lithotype and a number of the coal seam sampled are shown in tabular form (Table 1). These samples were taken in the years 2014–2015 by Bar-bara Bielowicz and Jacek Misiak. Since then all coal samples were stored in AGH in Krakow and therefore they could be used for the analyses, the results of which are included in this paper. The mentioned samples were ground to a grain size of <1 mm and used as material for polished sections made according to the ISO standard. The petrographic examination was carried out in both reflected white and blue light with the use of a Zeiss microscope and in accordance with the PN-ISO 7404-3:2001 standard. The maceral composition of both lignite and bituminous coal was determined according to the ICCP guidelines (Stach et al.
1982; Sýkorová et al. 2005). The macerals from the liptinite group were examined under
fluorescent light. The maceral nomenclature for the liptinite group was developed accor- ding to Pickel et al. (Pickel et al. 2017). The maceral group content analysis was carried out on 500 equally spaced points on the surface of the polished sections. The proximate analysis included content of: moisture, ash and volatile matter, as well as the gross calorific value determination. The ultimate analysis covered: carbon, hydrogen, nitrogen and sulfur
Table 1. Mine/złoże, age, lithotype and a number of the coal seam sampled Tabela 1. Kopalnia/złoże, wiek, litotyp i numer opróbowanego pokładu węgla
Sample location, age, lithotype
lignite
1l Pątnów (Jóźwin opencast), Middle Miocene (1st seam), detritic
2l Pątnów (Jóźwin opencast), Middle Miocene (1st seam), xylitic
3l Pątnów (Jóźwin opencast), Middle Miocene (1st seam), detro-xylitic
4l Turów, early Miocen (3rd seam), xylitic
5l Turów, early Miocen (3rd seam), xylo-detritic
7l Sieniawa, early-Middle Miocene (2nd seam), xylo-detritic
9l Sieniawa, early-Middle Miocene (2nd seam), detritic
10l Bełchatów, Early Miocen (3rd seam), xylo-detritic
12l Bełchatów, Early Miocen (3rd seam), detritic
13l Szczerców, early-Middle Miocene (2nd seam), xylitic
14l Szczerców, early-Middle Miocene (2nd seam), xylo-detritic
Bituminous coal
1B Janina, westphalian D (seam 119/2), bright coal 2B Janina, westphalian D (seam 119/2), dull coal 3B Janina, westphalian C (seam 203/2), bright coal 4B Janina, westphalian C (seam 203/2), dull coal 5B Bielszowice, Upper Namurian (seam 405/2), bright coal 6B Bielszowice Upper Namurian (seam 405/2), dull coal 7B Bielszowice, westphalian A (seam 510), bright coal 8B Bielszowice, westphalian A (seam 510), dull coal 9B Bogdanka, westphalian B (seam 391), bright coal 10B Bogdanka, westphalian B (seam 391), dull coal 11B Bogdanka, westphalian B (seam 381), bright coal 12B Bogdanka, westphalian B (seam 381), dull coal 15B Bogdanka, westphalian B (seam 385/2), bright coal 16B Bogdanka, westphalian B (seam 385/2), dull coal 17B Wesoła, Westphalian B (seam 308), bright coal 18B Wesoła, Westphalian B (seam 308), dull coal Coking coal 1CC Jas-Mos, westphalian A (seam 510), coking coal
content determination using a leco analyzer. All chemical and technological analyses were performed according to ISO standards. The analysis of major oxide composition of ash was determined by ICP-eS analysis in the Bureau Veritas laboratory. The total porosity was determined using an Autopore 9220 mercury porosimeter. Determining the spontaneous combustion indicator (modified Opaliński method) and calculating the activation indicator were carried out according to the Polish standard (PN-G-04558:1993). The two indicators of spontaneous ignition of coal and the activation energy indicator were determined. The ana- lysis of coal susceptibility to spontaneous combustion according to the PN-G-04558:1993 is based on the continuous measurement of the temperature of coal samples in the air stream at 510 K (237°C) and 463 K (190°C). Based on the continuous temperature measurement, the rate of its increment was determined, because, by assumption, it corresponds to the reaction rate. Both indicators of spontaneous ignition of coal were marked as Sza and Sza’. On their
basis, the energy of activation A was calculated. In order to classify coal based on the sus-ceptibility to spontaneous combustion, the indices Sza– the reaction rate determined
exper-imentally at temperature 237°C (°C/min) and A – the activation energy (kJ/mol) were used. Based on these two indicators, coal was classified into four self-combustion groups: V– very easily self-igniting (Sza – above 120°C/min), IV – easily self-igniting (Sza – 100–120°C/min,
A – 34–42 kJ/mol), III – moderately self-igniting (Sza – 80–100°C/min, A – 42–46 kJ/mol),
II – hardly self-igniting (Sza – below 80°C/min, A – 46-67 kJ/mol) and I – very hardly
self-igniting (Sza – below 80°C/min, A – above67 kJ/mol).
The impact of measured coal parameters on self-igniting and activation energy was de-termined using Pearson’s correlation analysis. All statistical analyses were performed using Statistica 12 software. The parameter distribution was determined using the t-test. In the first step, the correlation for the whole coal series, from lignite to anthracite, was examined. Then, these relationships were analyzed separately for low-, medium- and high-rank coals. The significance of the correlation was p = 0.05. The degree of correlation was determined according to Guilford (Guilford 1965).
2. results of lignite and bituminous coal analysis
2.1. proximate and ultimate analysis
The average, minimal and maximal values of the proximate and ultimate analysis of the 11 lignite, 16 bituminous coal, 1 coking coal and 1 antracite coal samples and are sum-marized in Table 2. The most variable parameter in the tested samples is ash oxide com-position. This is associated with various paleoenvironmental and diagenesis conditions under which the coal was formed. The ash content in lignite is highly variable. On the other hand, the moisture and sulfur content are the most variable for medium- and high-rank coal (Table 2).
Table 2. Basic statistics of proximate and ultimate analysis of coal
Tabela 2. Podstawowe statystyki parametrów technologicznych i chemicznych analizowanego węgla
Parameter All samples
Av Min Max Stdv V (%) Mad (%) 5.1 0.5 12.0 4.0 79.3 Adb (%) 10.7 1.2 46.2 9.4 87.3 Vdaf (%) 44.0 4.2 64.2 13.8 31.5 Stdb (%) 1.7 0.3 5.3 1.2 74.5 GCVdaf (MJ/kg) 30.3 23.9 36.4 4.0 13.2 Cdaf (%) 74.8 61.8 93.9 9.2 12.3 Hdaf (%) 5.1 3.2 6.0 0.5 9.8 Ndaf (%) 1.0 0.1 2.0 0.6 53.7 Odaf (%) 17.1 1.6 31.9 9.0 52.3 Total porosity (%) 12.6 1.7 35.9 11.5 91.0 Ash oxide composition (%)
SiO2 32.0 7.4 66.8 15.7 49.1 Al2O3 19.5 3.0 37.9 10.2 52.6 Fe2O3 14.4 2.2 58.9 12.2 84.8 MgO 3.3 0.3 12.4 3.2 98.8 CaO 11.5 1.0 30.0 9.1 79.4 Na2O 1.8 0.0 12.3 2.9 155.1 K2O 0.8 0.1 3.1 0.7 87.3 TiO2 1.0 0.2 2.9 0.6 63.3 P2O5 0.9 0.0 4.6 1.3 144.2 MnO 0.1 0.0 0.6 0.1 116.1 Cr2O3 0.1 0.0 0.2 0.0 67.7
Parameter low-rank coal
Av Min Max Stdv V (%) Mad (%) 9.1 4.6 12.0 1.9 21.2 Adb (%) 13.1 2.3 46.2 12.7 96.5 Vdaf (%) 57.2 49.4 64.2 3.6 6.3 Stdb (%) 1.9 0.5 3.9 1.2 63.5 GCVdaf (MJ/kg) 26.4 23.9 29.2 1.5 5.6 Cdaf (%) 65.9 61.8 70.7 2.3 3.5 Hdaf (%) 5.3 4.7 6.0 0.4 7.5 Ndaf (%) 0.5 0.1 0.9 0.3 55.2 Odaf (%) 26.0 21.5 31.9 2.9 11.3 Total porosity (%) 24.6 8.4 35.9 8.3 33.7 Ash oxide composition (%)
SiO2 31.8 7.4 66.8 21.0 66.2
Al2O3 11.2 3.0 19.0 5.7 50.9
Table 2. cont. Tabela 2. cd.
Parameter low-rank coal
Av Min Max Stdv V (%) MgO 3.1 0.3 11.0 2.9 95.3 CaO 16.9 1.2 30.0 10.1 60.0 Na2O 1.8 0.0 12.3 4.0 217.3 K2O 0.5 0.1 1.2 0.4 73.7 TiO2 1.1 0.3 2.9 0.9 80.6 P2O5 0.1 0.0 0.3 0.1 79.5 MnO 0.1 0.0 0.2 0.1 65.4 Cr2O3 0.1 0.0 0.2 0.1 75.6
Parameter Medium- and high-rank coal
Av Min Max Stdv V (%) Mad (%) 1.9 0.5 5.1 1.6 85.7 Adb (%) 8.4 1.2 24.1 5.3 63.7 Vdaf (%) 33.6 4.2 40.5 8.9 26.6 Stdb (%) 1.4 0.3 5.3 1.2 86.4 GCVdaf (MJ/kg) 33.3 29.0 36.4 2.3 6.8 Cdaf (%) 81.8 74.1 93.9 5.6 6.8 Hdaf (%) 4.9 3.2 5.4 0.5 10.7 Ndaf (%) 1.5 1.0 2.0 0.3 21.6 Odaf (%) 10.3 1.6 19.3 5.2 50.4 Total porosity (%) 4.1 1.7 10.5 2.5 61.3 Ash oxide composition (%)
SiO2 32.4 9.6 50.7 11.9 36.8 Al2O3 25.6 8.7 37.9 8.6 33.7 Fe2O3 17.7 3.6 58.9 14.9 84.6 MgO 3.5 0.4 12.4 3.5 100.6 CaO 7.2 1.0 19.8 5.9 82.0 Na2O 2.0 0.4 6.3 2.0 99.7 K2O 1.0 0.2 3.1 0.8 74.9 TiO2 1.0 0.2 1.5 0.5 46.9 P2O5 1.5 0.0 4.6 1.5 98.9 MnO 0.1 0.0 0.6 0.2 129.4 Cr2O3 0.0 0.0 0.1 0.0 39.7
Mab – moisture content, analytical basis, Adb – ash content, dry basis, Vdaf – volatile matter content, dry ash-free
basis, Stdb – total sulfur content, dry basis, GCVdaf – gross calorific value, dry ash-free basis, Cdaf – carbon
content, dry ash-free basis, Hdaf – hydrogen content, dry ash-free basis, Ndaf – nitrogen content, dry ash-free basis,
Odaf – oxygen content, dry ash-free basis.
2.2. petrographic analysis
The average, minimum and maximum values of the petrographic analyses of the 29 lig-nite and bituminous coal samples are presented in Table 3. The examined liglig-nite samples are dominated by the huminite group, which shows the lowest variability. Among the macerals of this group, textinite has the highest variability. In the case of low-rank coals, the most variable components are macerals of the inertinite group, in particular fusinite and semi-fusinite. The composition of bituminous coal is moderately variable (Table 3). The diffe- rences in the content of individual maceral groups are due to the fact that the samples include both bright and dull coal lithotypes.
Table 3. Basic statistics of the petrographic composition (macerals content) of the examined lignites and bituminous coals
Tabela 3. Podstawowa statystyka składu petrograficznego (zawartości macerałów) w badanym węglu brunatnym i kamiennym
lignite
Parameter (%) Av Min Max Stdv V (%) Textinite 18.2 1.3 82.7 27.5 150.6 Ulminite 21.3 2.4 55.2 16.2 76.1 Attrinite 20.8 2.0 49.9 13.4 64.4 Densinite 26.7 2.0 46.2 14.3 53.4 Corpohuminite 0.1 0.0 0.8 0.3 200.8 Gelinite 2.3 0.0 8.6 2.5 111.0 Sporinite 0.5 0.2 1.0 0.3 55.9 Cutinite 0.1 0.0 0.3 0.1 61.1 Resinite 1.4 0.5 2.1 0.6 46.3 Suberinite 0.2 0.0 0.6 0.2 104.4 Alginite 0.1 0.0 0.3 0.1 150.0 liptodetrinite 0.4 0.0 1.1 0.3 91.5 Fusinite 1.8 0.0 14.0 3.8 204.6 Semifusinite 0.1 0.0 0.8 0.2 260.6 Funginite 0.1 0.0 0.5 0.2 246.1 Inertodetrinite 1.1 0.0 4.7 1.5 128.7 Pyrite 0.4 0.0 1.6 0.5 149.8 Carbonates 0.0 0.0 0.4 0.1 360.6 Quartz + Clays 4.0 0.4 10.3 3.0 75.2 Huminite 89.4 73.6 97.1 7.4 8.3 Inertinite 3.2 0.0 19.9 5.3 164.5
Table 3. cont. Tabela 3. cd.
lignite
Parameter (%) Av Min Max Stdv V (%) liptinite 3.0 1.7 8.5 1.8 59.9 Mineral matter 4.4 0.4 11.1 3.2 72.9
Ro 0.3 0.2 0.3 0.0 6.5
GI 1.7 0.1 4.0 1.3 74.2
Bituminous coal
Parameter (%) Av Min Max Stdv V (%) Telinite 0.6 0.0 2.2 0.6 104.5 Collotelinite 15.8 2.2 100.0 23.1 146.5 Collodetrinite 43.9 0.0 76.8 19.6 44.7 Vitrodetrinite 1.3 0.0 5.8 1.5 116.9 Corpogelinite 1.2 0.0 3.6 1.1 91.9 Gelinite 1.2 0.0 3.4 1.0 81.2 Macrosporinite 1.4 0.0 7.8 1.8 132.1 Microsporinite 7.8 0.0 16.4 4.7 60.6 Cutinite 1.0 0.0 4.6 1.1 107.1 Resinite 0.7 0.0 2.2 0.6 84.0 liptodetrinite 2.8 0.0 10.4 3.3 120.1 Fusinite 4.1 0.0 18.5 4.2 101.3 Semifusinite 3.2 0.0 8.6 2.4 75.4 Funginite 4.2 0.0 10.4 3.1 73.1 Macrinite 1.3 0.0 4.0 1.4 114.2 Micrinite 0.3 0.0 1.0 0.3 92.0 Secretinite 0.1 0.0 0.2 0.1 115.0 Inertodetrinite 7.3 0.0 17.6 5.0 67.6 Sulfides 0.7 0.0 3.8 0.9 140.8 Carbonates 0.0 0.0 0.4 0.1 424.3 Clay minerals 1.1 0.0 6.2 1.5 137.3 Vitrinite 64.1 40.0 100.0 17.7 27.6 Inertinite 20.5 0.0 38.2 10.8 52.9 liptinite 13.7 0.0 28.2 9.6 70.1 Mineral matter 1.8 0.0 7.6 1.8 103.5 Ro 0.7 0.5 2.3 0.4 55.3 GI 6.7 1.8 30.4 7.4 109.3
2.3. analysis of spontaneous combustion tendency
The spontaneous combustion parameters of lignites and bituminous coals were averaged (Tables 4 and 5). The examined Polish lignites are characterized by lower rates of sponta-neous combustion in 237°C (Sza) and 190°C (Sza’) and a higher activation energy of carbon
oxidation (A) than bituminous coals. The obtained values can be used when determining the spontaneous combustion tendency groups according to the PN-G-04558:1993 standard. On average, the examined lignites represent the first spontaneous combustion tendency group (coals with very low tendency to spontaneous combustion) and the examined bituminous coals – the second group (coals with low tendency to spontaneous combustion). The analysis has shown that the greatest variability among the tested samples was observed in the case of spontaneous combustion at 190°C. However, this variability should be considered as mod-erate (Mucha 1994).
From all of the examined lignites, the xylitic lignite from the Szczerców deposit, with the lowest rates of spontaneous combustion and the highest activation energy of carbon oxidation, is the least prone to spontaneous combustion. Lignites most susceptible to self-ignition include: xylo-detritic lignite from the Sieniawa deposit, characterized by the highest rate of spontaneous combustion in 237°C, xylitic lignite from the Turów deposit,
Table 4. Basic statistics of spontaneous combustion in 237°C, spontaneous combustion in 190°C, activation energy A and self-ignition group
Tabela 4. Podstawowa statystyka skłonności do samozapłonu w temp. 237°C, skłonności do samozapłonu w temp. 190°C, energii aktywacji A oraz grup samozapalności
Parameter All samples
Av Min Max Stdv V (%) Spontaneous combustion in 237°C Sza (°C/min) 70.2 16.0 125.0 27.4 39.0
Spontaneous combustion in 190°C Sza’ (°C/min) 15.4 3.0 38.4 9.7 62.8
Activation energy A (kJ/mol) 69.8 47.5 96.8 16.3 23.4 low-rank coal
Spontaneous combustion in 237°C Sza (°C/min) 66.1 35.8 92.2 19.8 29.9
Spontaneous combustion in 190°C Sza’ (°C/min) 9.2 3.7 14.7 4.2 45.7
Activation energy A (kJ/mol) 85.4 66.6 95.4 9.5 11.1 Medium- and high-rank coal
Spontaneous combustion in 237°C Sza (°C/min) 72.7 16.0 125.0 31.4 43.2
Spontaneous combustion in 190°C Sza’ (°C/min) 19.2 3.0 38.4 10.2 53.1
Table 5. Spontaneous combustion parameters of the examined coals Tabela 5. Parametry samozapłonu badanych węgli
Sample Spontaneous combustion in 237°C Sza (°C/min) Spontaneous combustion in 190°C Sza’ (°C/min) Activation energy A (kJ/mol) Self-ignition group 1l 77.90 9.00 90.70 1 2l 40.10 4.30 93.90 1 3l 69.80 8.00 90.90 1 4l 85.70 14.70 74.20 3 5l 78.50 11.50 80.60 1 7l 92.20 13.80 79.80 3 9l 47.60 5.20 93.30 1 10l 70.40 14.40 66.60 2 12l 47.60 5.20 93.30 1 13l 35.80 3.70 95.40 1 14l 81.40 11.80 81.10 3 1B 43.30 9.00 66.00 2 2B 125.00 32.00 57.30 5 3B 118.80 38.40 47.50 4 4B 114.30 31.70 54.00 4 5B 43.10 10.00 61.40 2 6B 41.00 7.40 72.20 1 7B 55.40 13.40 59.70 2 8B 50.40 14.50 52.50 2 9B 78.90 23.60 50.70 2 10B 78.60 20.60 56.30 2 11B 91.50 27.10 51.10 3 12B 101.00 30.60 50.20 4 15B 87.20 23.20 55.70 3 16B 87.20 22.50 57.00 3 17B 67.70 16.30 59.80 2 18B 78.80 16.00 67.10 1 1CC 31.00 6.10 68.50 1 1A 16.00 3.00 96.80 1
which has the highest rate of spontaneous combustion in 190°C and xylo-detritic lig-nite from the Bełchatów deposit, with the lowest activation energy of carbon oxidation (Table 5).
In the case of bituminous coals and anthracite, the examined anthracite sample 1A is the least susceptible to self-ignition – it has the lowest rate of spontaneous combustion, and the highest activation energy of carbon oxidation. The coals from the Janina mine are the most prone to spontaneous combustion. The dull coal from seam 119/2 has the highest rate of spontaneous combustion in 237°C, while the bright coal from seam 203/3 has the highest rate of spontaneous combustion in 190°C and the lowest activation energy of carbon oxida-tion (compare Tables 1 and 5).
3. correlation analysis
The correlation analysis between the ultimate and proximate analysis and the self-igni-tion indices has been carried out for all samples and separately for lignite and bituminous coal. Pearson’s linear correlation coefficients, which are statistically significant at p = 0.05, are presented in Table 6. The following interpretation of correlation coefficients was used for r (Guilford 1965):
(0.00–0.20) weak correlation, the relationship is almost insignificant,
(<0.20–0.40) low correlation, the relationship is clearly visible,
(<0.40–0.70) strong correlation, the relationship is clearly visible and small,
(<0.70–0.90) strong correlation, significant relationship,
(<0.90–1.00) both correlation and relationship are strong.
Considering the results obtained, it can be stated that more correlations are observed for all rank coal than for lignite and bituminous coal separately. The strongest correlations are observed between measures of the rank of coal (Vdaf, GCVdaf, Cdaf, Odaf) and the activation
energy A. An example of a negative moderate relationship is between gross calorific value and activation energy (Fig. 1). It should also be noted that anthracite is characterized by both high activation energy and high gross calorific value. A significant negative relationship between the nitrogen content in coal and the activation energy was also observed (Fig. 2). The total porosity and activation energy are strongly positively correlated (Fig. 3). The oxide composition of ash has a moderate effect on the susceptibility of coal to self-ignition. The Al2O3 content in ash has the strongest impact on the activation energy. One of the main ob-jectives of the study was to find the relationship between the petrographic composition and the susceptibility to self-ignition of coal. Based on the results of presented analysis, it can be stated that this relationship is clearly visible. Semifusinite, the total amount of inertinite and liptinite macerals have the greatest effect on Sza, Sza’ and A, respectively (Figs. 4–6). These
components are moderately negatively correlated with the activation energy. At the same time, the activation energy increases with increasing amount of macerals of the huminite/ /vitrinite group (Fig. 7).
Table 6. Pearson’s correlation coefficient for lignite and bituminous coal parameters. In bold – strong and very strong correlation
Tabela 6. Współczynnik korelacji Pearsona dla parametrów węgla brunatnego i węgla kamiennego. Pogrubienie – silna i bardzo silna korelacja
All samples lignite Bituminous coal Variable (°C/min)Sza (°C/min)Sza’ (kJ/mol)A (°C/min)Sza (°C/min)Sza’ (kJ/mol)A (°C/min)Sza (°C/min)Sza’ (kJ/mol)A
Ultimate and proximate analysis
Mad (%) 0.67 –0.64 0.61 0.53 0.49 Vdaf (%) 0.44 –0.60 0.69 0.61 –0.81 GCVdaf (MJ/kg) –0.61 –0.59 –0.50 Cdaf (%) –0.52 –0.70 –0.61 0.57 Hdaf (%) 0.48 0.47 –0.83 N daf (%) 0.55 –0.79 –0.50 Odaf 0.58 0.62 0.53 Sza (°C/min) 0.88 –0.55 0.90 –0.69 0.97 –0.72 Sza’ (°C/min) 0.88 –0.81 0.90 –0.93 0.97 –0.77 A (kJ/mol) –0.55 –0.81 –0.69 –0.93 –0.72 –0.77 Total porosity (%) –0.42 0.73 Al2O3 0.56 –0.69 CaO –0.48 –0.55 0.55 K2O 0.48 0.58 –0.49 0.49 P2O5 –0.47 Petrographic analysis Telinite 0.39 Collotelinite 0.72 Collodetrinite –0.56 –0.47 Vitrodetrinite –0.40 0.62 Corpogelinite 0.48 –0.51 Cutinite 0.47 0.62 –0.55 0.53 0.55 liptodetrinite 0.42 0.45 Semifusinite 0.50 0.70 –0.69 0.61 0.60 –0.48 Funginite –0.55 Inertodetrinite –0.51 Huminite/vitrinite (%) –0.48 0.62 Inertinite (%) –0.56 Liptinite (%) 0.48 0.58 –0.61 0.54 Mineral matter (%) Random reflectance (%) –0.37 –0.61 –0.55 0.85 Gelification index –0.46 0.62
Fig. 1. The relationship between the gross calorific value and the activation energy. Coal sample numbers as in Table 1
Rys. 1. Zależność między ciepłem spalania a energią aktywacji. Numery próbek węgla jak w tabeli 1
Fig. 2. The relationship between nitrogen content in coal and the activation energy. Coal sample numbers as in Table 1
Rys. 2. Zależność między zawartością azotu w węglu a energią aktywacji. Numery próbek węgla jak w tabeli 1
Fig. 3. The relationship between the total porosity in coal and the activation energy. Coal sample numbers as in Table 1
Rys. 3. Zależność między całkowitą porowatością węgla a energią aktywacji. Numery próbek węgla jak w tabeli 1
Fig. 4. The relationship between semifusinite content in coal and the activation energy. Coal sample numbers as in Table 1
Rys. 4. Zależność między zawartością semifuzynitu w węglu a energią aktywacji. Numery próbek węgla jak w tabeli 1
Fig. 5. The relationship between the content of macerals of the inertinite group in coal and the activation energy. Coal sample numbers as in Table 1
Rys. 5. Zależność między zawartością macerałów z grupy inertinitu w węglu a energią aktywacji. Numery próbek węgla jak w tabeli 1
Fig. 6. The relationship between the content of macerals of the liptinite group in coal and the activation energy. Coal sample numbers as in Table 1
Rys. 6. Zależność między zawartością macerałów z grupy liptinitu w węglu a energią aktywacji. Numery próbek węgla jak w tabeli 1
In the case of lignite, high correlations between the chemical, technological, petrograph-ic properties and the self-combustion parameters are rare. Moderate correlations were ob-served only in the case of moisture and volatile matter content. Moisture (Mad) correlates
positively with the activation energy of carbon oxidation (A) and negatively with rates of spontaneous combustion in 190°C (Sza’). This suggests that lignite with higher moisture
content has a lower tendency of self-combustion. A significant negative correlation between the volatile matter content (Vdaf) and the rate of spontaneous combustion in 237°C (Sza) has
been observed. This suggests that in the case of lignite, the rate of spontaneous combustion decreases with increasing the volatile matter content (Table 6).
In the case of bituminous coals and anthracite, moderate and high correlations were ob-served between all of the self-combustion parameters and the volatile matter content (Vdaf).
The moderate positive correlations with the rates of spontaneous combustion (Sza, Sza’) and
the high negative correlation with the activation energy of carbon oxidation (A) suggest that the bituminous coal tendency to self-combustion increases with the increasing volatile mat-ter content. A similar dependence was observed in the case of the hydrogen (Hdaf) and
oxy-gen (Odaf) content. However, some of the correlations between them and the self-combustion
parameters were moderate. Other important correlations were observed in the case of the moisture content and in the case of carbon content (Cdaf). The values of Pearson’s correlation
Fig. 7. The relationship between the content of macerals of the huminite/vitrinite group in coal and the activation energy. Coal sample numbers as in Table 1
Rys. 7. Zależność między zawartością macerałów z grupy huminitu/witrynu w węglu a energią aktywacji. Numery próbek węgla jak w tabeli 1
coefficient suggest that higher moisture contributes to the higher tendency to self-combu- stion and that a high carbon content is the reason for lower tendency of self-combustion. The relationship between carbon content and self-ignition is also consistent with the correlation between the self-combustion parameters and the reflectance (Ro) – another parameter of the degree of coalification. Both the Cdaf and Ro correlation suggest that in the case of
bitumi-nous coals, the coal with a higher degree of coalification is less prone to self-combustion. Taking into account the correlations with the petrographic composition, the most noticeable are those concerning the content of macerals from the collotelinite, semifusinite and liptinite groups. The collotelinite content has a high, positive correlation with the activation energy of carbon oxidation, which suggests that the high content of this maceral is related to the lower tendency to self-combustion. Semifusinite has moderate to high positive correlations with the rates of spontaneous combustion and a moderate, negative correlation with the the activation energy of carbon oxidation. In this case, the results suggest that the high semi-fusinite content is responsible for the higher tendency to self-combustion. The situation is similar in the case of macerals from the liptinite group, where their higher content contri- butes to the higher tendency to self-combustion (Table 6).
Comparing the correlation results obtained for lignite and bituminous coal, it was noted that the most noticeable correlations were reverse. This means that the correlation between the activation energy of carbon oxidation (A) and volatile matter content (Vdaf) was positive
in the case of lignite and negative in the case of bituminous coal. Those differences may result from the different molecular composition of lignite and bituminous coal.
In the case of lignite, the increase of carbon content is the reason for its higher tendency to self-ignition. Carbon in lignite is mostly connected by aliphatic bonds. Their high concen-tration in coals contributes to their instability and susceptibility to oxidation and self-heat-ing. In the case of different bituminous coals, the content of aliphatic bonds decreases with the degree of coalification, which may explain the observed correlation between the decreas-ing tendency to spontaneous combustion and the increasdecreas-ing carbon content (Berkowitz and Schein 1951; Choudhury et al. 2016).
Numerous authors have suggested that the susceptibility to spontaneous combustion in-creases with the increase in the moisture content (Ciesielczuk et al. 2013; wen et al. 2015; Choudhury et al. 2016). This is consistent with the correlation results for bituminous coal, but not for lignite. However, some researchers believe that the high moisture content may inhibit the self-heating process (Bhattacharyya 1972; Choudhury et al. 2016). In addition, a study by Xu et al. (Xu et al. 2013) has shown that the critical moisture content in lignite, when it is the most prone to self-ignition, is 25% (Xu and He 2013). The obtained correlation results suggest that in the case of lignite the lower tendency to self-combustion decreases with the increase of moisture content. In the examined samples, the average total moisture content was 18.59%, that is, lower than the critical moisture content. In this case, it can be said that the moisture inhibits the self-ignition.
Numerous authors have noted that the petrographic composition affects the tendency of coal to spontaneous combustion. Simply put, the reactive macerals from the vitrinite and
liptinite groups are the reason for the higher susceptibility to self-ignition (Falcon 1986; Chakravorty and Kolada 1988; Misra and Singh 1994; Mastalerz et al. 2011; Misz-Kennan and Fabiańska 2011) i.e., the maceral and microlithotype composition of the organic matter and its rank. Alteration of wastes also depends on the heating history, i.e., the rate of heat-ing, final heating temperature, duration of heatheat-ing, and the degree of air access. Although air is probably necessary to initiate and drive the heating processes, these usually take place under relatively oxygen depleted conditions. With slow heating, color of organic matter par-ticles changes, irregular cracks and oxidation rims develop around edges and cracks, and bitumen is expelled. As a result, massive and detritic isotropic and strongly altered organic matter forms. On the other hand, higher heating rates cause the formation of devolatilization pores, oxidation rims around these pores and along cracks, vitrinite-bands-mantling parti-cles, and bitumen expulsions. Organic compounds generated from the wastes include n-al-kanes, iso-aln-al-kanes, alkylcyclohexanes, acyclic isoprenoids, mainly pristane, phytane and, in some cases, farnesane, sesquiterpanes, tri- and tetracyclic diterpanes, tri- and pentacyclic triterpanes, and steranes, polycyclic aromatic hydrocarbons (mostly with two- to five rings, rarely six rings. The correlation analysis of bituminous coal also suggests the influence of liptinite content on spontaneous combustion parameters. However, in the case of vitrinite, no significant correlation has been observed. Only collotelinite content correlates positively with the activation energy of carbon oxidation, which suggests that this maceral makes coal less prone to self-ignition. However, this observation may be biased by the results obtained for anthracite, which consisted of collotelinite only, and because of the fact that its mole- cular composition is the least susceptible to spontaneous combustion of all of the examined coals. The correlation analysis also suggests that the increase of the semifusinite content makes coal more prone to self-ignition. This observation may be connected to the reactive inertinite, which is represented by low-reflectance micrinite and semifusinite (Kruszewska 1989; Kruszewska and Misz 1999). In the case of vitrinite and liptinite, their impact on the combustion tendency of coal is related to their reactivity. The obtained correlation results may suggest that the situation is similar in the case of semifusinite.
conclusions
The results of analyses included in this paper partially confirmed the investigation re-sults of other researchers. It should be noted that previous studies have not taken the litho-type of coal into account. Generally, it can be stated that there is no data in the litera-ture on petrography of lignite. The current studies have shown that the mixed lithotypes (xylo-detritic and detro-xylitic coal) have the highest tendency of spontaneous combustion. The correlation relationships between ultimate and proximate analysis, the energy of acti-vation and self-ignition are more pronounced in bituminous coal. The influence of moisture, volatile matter content, carbon and hydrogen content on the tendency to self-combustion and activation energy is clearly visible. This is observed both for lignite and bituminous coal.
In addition, it has been shown that the tendency to self-heating decreases with the increas-ing degree of coalification. No relationship between the pyrite content and the tendency to self-ignition has been confirmed. The obtained results are particularly important in the context of gasification. In the case of underground gasification, it is particularly important to prevent underground fires in the voids after gasification. This is due to the occurrence of the unreacted, dried coal with a low volatile matter content. In the context of the research results presented in this paper, it can be concluded that the ungasified coal left in the voids has a lower tendency to self-combustion than coal before the gasification process.
The author would like to thank Jacek Misiak, PhD for performing petrographic analysis on bitu-minous coal samples and the reviewers for insightful and accurate comments. This paper was finan-cially supported by the Polish National Science Centre (NCN), under research project awarded by decision no. DEC-2013/09/D/ST10/04045, from subsidy no 16.16.140.315.
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changes in the spontaneous combustion tendency of humic coals according to their petrographic composition and physico-chemical properties
K e y wo r d s
coal, spontaneous combustion, self-ignition, activation energy, petrography, A b s t r a c t
The article presents the analysis of the correlation between the self-ignition parameters and the ultimate and proximate analysis, as well as the petrographic properties of Polish lignite, sub-bitumi-nous coal, bitumisub-bitumi-nous coal and anthracite. The following coal properties were determined: the mo-isture, ash content, volatile matter and sulfur content, gross calorific value, net calorific value, C, O, N and H contents, total porosity, ash oxide composition, rates of spontaneous combustion in 237°C and 190°C, and activation energy. During the petrographic analysis, maceral composition and random reflectance were determined. To determine the linear correlation between the self-ignition parame-ters and the analyzed coal properties, the Pearson correlation coefficient was calculated. The results show that there is no strong linear correlation between the lignite tendency to self-ignition and its petrographic properties. However, a strong negative correlation between the rate of spontaneous com-bustion and moisture and volatile matter content was observed. In the case of bituminous coal, strong correlations between self-combustion parameters and various coal properties were confirmed. The most noteworthy are the correlations between self-ignition parameters and the maceral composition, that is, between the content of macerals of the inertinite group in coal and content of macerals of the huminite/vitrinite group. The obtained results suggest that the spontaneous combustion tendency of coal increases with the increasing content of semifusinite and liptinite.
WpłyW parametróW fizyko-chemicznych i petrograficznych Węgla humusoWego na jego skłonność do samozapłonu
S ł ow a k l u c z owe
węgiel, samozapłon, samozagrzewanie, energia aktywacji, petrografia S t r e s z c z e n ie
W artykule przedstawiono analizę korelacji między parametrami samozapłonu a analizą tech-nologiczną i chemiczną oraz właściwościami petrograficznymi polskiego węgla brunatnego, węgla kamiennego i antracytu. Określono następujące właściwości węgla: wilgotność, zawartość popiołu, zawartość części lotnych i siarki, wartość opałową, ciepło spalania, zawartość C, O, N i H, całkowi-tą porowatość, skład tlenkowy popiołu, wskaźniki samozapłonu w temperaturach 237 i 190°C oraz energię aktywacji. Podczas analizy petrograficznej określono skład petrograficzny i współczynnik odbicia. W celu określenia liniowej korelacji między parametrami samozapłonu a analizowanymi właściwościami węgla obliczono współczynnik korelacji Pearsona. Uzyskane wyniki pokazują,
że nie ma silnej korelacji liniowej między tendencją węgla do samozapłonu a jego właściwościami petrograficznymi. Zaobserwowano jednak silną ujemną korelację między samozapłonem a wilgotno-ścią i zawartowilgotno-ścią substancji lotnych. W przypadku węgla kamiennego potwierdzono silne korelacje między parametrami samozapłonu a różnymi właściwościami węgla. Najbardziej godne uwagi są ko-relacje między parametrami samozapłonu a składem petrograficznym, tj. pomiędzy zawartością ma-cerałów grupy inertynitu w węglu oraz zawartością mama-cerałów grupy huminitu/witrynitu w węglu. Uzyskane wyniki sugerują, że tendencja do samozapłonu węgla rośnie wraz ze wzrostem zawartości semifuzynitu i liptynitu.
